Coatings are frequently applied to the surfaces of glass sheets to provide the glass sheets with desirable characteristics. The coatings applied to the glass sheets vary widely and may include low-emissivity coatings, photocatalytic coatings, anti-reflective coatings, hydrophobic coatings, or hydrophilic coatings. Further, a coating may be applied simply to impart a specific color to the glass sheet.
A low emissivity coating may be applied to a glass sheet to reduce the passage of infrared radiation through the glass. This reduces loss or gain of heat through glass, thereby enhancing the ability to control the temperature in the building. Low-emissivity coatings are well known in the art and typically include one or more layers of infrared-reflective metal and one or more transparent dielectric layers. The infrared-reflective layers, which are typically conductive metals such as silver, gold, or copper, reduce the transmission of radiant heat through the coating. The transparent dielectric layers are used primarily to reduce visible reflectance and to control other properties of the coatings, such as color. Commonly used transparent dielectrics include oxides of zinc, tin, indium, bismuth, and titanium, and alloys and mixtures thereof, as well as certain nitrides (e.g., silicon nitride and titanium nitride). Low-emissivity coatings are commonly deposited on glass substrates through the use of well known magnetron sputtering techniques.
Photocatalytic coatings may be applied to glass sheets to provide self-cleaning characteristics to the glass. A photocatalytic coating applied to the outer surfaces of a glass sheet window reduces the time and cost associated with cleaning the outer surface of the window. The field of photocatalytic coating technology is founded on the ability of certain materials to absorb radiation and photocatalytically degrade organic materials such as oil, plant matter, fats, and greases. The most powerful of these photocatalytic materials appears to be titanium oxide. However, other materials are believed to exhibit photoactivity as well. These materials include oxides of iron, silver, copper, tungsten, aluminum, zinc, strontium, palladium, gold, platinum, nickel, and cobalt. Useful photocatalytic coatings are described in U.S. Pat. No. 5,874,701 (Watanabe et al), U.S. Pat. No. 5,853,866 (Watanabe et al), U.S. Pat. No. 5,961,843 (Hayakawa et al.), U.S. Pat. No. 6,139,803 (Watanabe et al), U.S. Pat. No. 6,191,062 (Hayakawa et al.), U.S. Pat. No. 5,939,194 (Hashimoto et al.), U.S. Pat. No. 6,013,372 (Hayakawa et al.), U.S. Pat. No. 6,090,489 (Hayakawa et al.), U.S. Pat. No. 6,210,779 (Watanabe et al), U.S. Pat. No. 6,165,256 (Hayakawa et al.), and U.S. Pat. No. 5,616,532 (Heller et al.), the entire contents of each of which are incorporated herein by reference.
Hydrophobic coatings are applied to glass to repel water, thus causing the water to bead up, rather than spreading into a sheet. U.S. Pat. No. 5,424,130 to Nakanishi, et al., the teachings of which are incorporated herein by reference, suggests coating a glass surface with a silica-based coating which incorporates fluoroalkyl groups. The reference teaches applying a silicone alkoxide paint onto the surface of the glass, drying the paint and then burning the dried paint in air.
Hydrophobic (i.e., “water repellent”) coatings tend to cause water on the surface of the glass to bead up. If the coating is applied to an automobile windshield or the like where a constant flow of high velocity air is blowing over the surface, this water beading effect can help remove water from the glass surface by allowing the droplets to blow off the surface. However, in more quiescent applications, these droplets will tend to sit on the surface of the glass and slowly evaporate. As a consequence, this supposed “water repellent” coating will not solve the water-related staining problems noted above. To the contrary, by causing the water to bead up more readily, it may actually exacerbate the problem.
Thus, it may be desirable to produce glass bearing a hydrophilic coating. Hydrophilic coatings have an affinity for water and tend to cause water applied thereto to sheet. As described in U.S. patent application Ser. Nos. 09/868,542, 09/868,543, 09/599,301, and 09/572,766, the entire contents of each of which are incorporated herein by reference, hydrophilic coatings may be particularly advantageous when used on architectural glass and other substrates. For example, these coatings may resist formation of stains left by sitting water droplets, thereby promoting a longer lasting clean appearance.
Antireflective coatings may also be applied to the surface of a glass sheet. For example, U.S. Pat. No. 5,394,269 to Takamatsu, et al., the entire teachings of which are incorporated herein by reference, proposes a “minutely rough” silica layer on the surface of glass to reduce reflection. The roughened surface is achieved by treating the surface with a supersaturated silica solution in hydrosilicofluoric acid to apply a porous layer of silica on the glass sheet.
It is conventional to apply coating entirely over the coated surface of glass sheets used for architectural or automotive applications. Glass sheets can be coated using a variety of different coating methods. Sputter deposition is a large area coating method that is well suited for the application of thin films. Sputtering is fairly conventional in the architectural and automotive glass industries. For example, magnetron sputtering equipment and processes are well known in the present art. Magnetron sputtering chambers and methods are described in U.S. Pat. No. 4,166,018 (Chapin), the entire teachings of which are incorporated herein by reference.
As noted above, low-emissivity coatings typically comprise one or more infrared-reflective metallic layers. These metallic layers are commonly formed of silver, which is quite vulnerable to chemical attack. For example, silver is known to corrode when exposed to oxygen and moisture. When the silver in a low-emissivity coating corrodes, there is typically an attendant degradation of coating quality. For example, corrosion of the silver in a low-emissivity coating may reduce the infrared reflectivity of the coating, hence jeopardizing its intended function. This corrosion may also negatively impact the aesthetic appearance of the coated article. As a result, low-emissivity coatings are typically limited to use on the inner surfaces of multiple-pane insulating glass units (i.e., IG units), where these coatings are protected from the ambient environment.
Substrates bearing interior low-emissivity coatings are preferably edge deleted before being incorporated into IG units. A typical double-glazed IG unit comprises two panes held in a spaced-apart relationship by a spacer. The confronting, inner surfaces of the panes define between them a sealable between-pane space. Commonly, the inner surface of one of the panes bears a low-emissivity coating.
Low-emissivity coatings are typically less than ideal for bonding with a spacer. As noted above, these coatings tend to lack chemical stability. This makes it difficult to durably bond a spacer to a surface bearing such a coating. For example, when the infrared-reflective material in a low-emissivity coating corrodes, it may be difficult to form or maintain a strong bond with the corroded surface. Thus, to provide durable bonding of the spacer to the thus coated surface, it is advantageous to remove the low-emissivity coating from the area of the inner pane surface to which the spacer will be bonded. This process is referred to as “edge deletion”.
It is known to perform edge deletion of interior low-emissivity coatings. In this regard, reference is made to U.S. Pat. No. 4,716,686 (Lisec) and U.S. Pat. No. 5,934,982 (Vianello et al.), the entire teachings of each of which are incorporated herein by reference.
Unlike interior low-emissivity coatings, exterior coatings typically do not suffer from the corrosion problems discussed above. Thus, edge deletion has traditionally not been performed on exterior coatings. However, it would be advantageous to perform edge deletion of exterior coatings as well as interior coatings. Thus, it would be desirable to provide methods and equipment for removing coatings from both major surfaces of a glass sheet, particularly if both coatings could be removed simultaneously.